The invention relates to a method and apparatus for improving the quality of marine seismic signals. Such signals are gathered by using an array of marine seismic sources which is towed behind a vessel. The array comprises a plurality of subarrays, and each subarray carries seismic sources. The marine seismic method is a generally known geophysical exploration method to investigate the character of submarine earth formations and, more particularly, to survey subterranean formations. According to such seismic method, shocks at preselected submarine locations are generated periodically by marine seismic sources, and various characteristics of the shock waves, causing seismic waves as they enter the earth and being reflected or refracted from submarine earth strata, are measured. These shock waves are detected by sensitive instruments, placed at varying distances from the shock generation locations, and are subsequently converted into electrical signals which may be further processed to derive data that may be plotted to obtain a picture or map of the investigated area. These plots may indicate the presence, if any, of structural traps capable of holding hydrocarbons like oil and/or gas.
The amplitude of the shock waves of interest is a function of the size of the shock, and it will be clear that a wave having a relatively thin amplitude may be studied easier than a wave having a lesser amplitude. It has now become a usual practice in marine seismic exploration to use nondynamite seismic sources, for example, airguns. The following description will relate to airguns only, but it will be clear to those skilled in the art that every suitable marine seismic source can be used. An example of such a suitable seismic source is a water-gun. The airguns are towed behind a marine vessel by means of a towing cable and may suddenly release confined volumes of high pressure gas, thus emitting acoustical pressure pulses having a certain acoustic energy which cause seismic waves as they enter the earth. The acoustic signal, generated by an airgun, shows a series of pressure pulses resulting in a corresponding oscillatory frequency spectrum having a fundamental frequency which is related to the depth of the airgun and the amount and pressure of released gas volume. Moreover, airguns are low intensity energy sources, so that the acoustic energy waves produced by an airgun will have low amplitude, and any electrical signal derived therefrom will have a relatively low signal-to-noise ratio. The signal strength produced by an airgun is several orders of magnitude less than that generated by a dynamite explosion or other suitable explosives. These features make a single airgun less effective as a seismic source. Moreover, marine seismic exploration technology requires a way of detecting seismic waves, which involves added noise, because the detectors cannot normally be fixedly located so as to detect a number of shocks of equal significance. Instead, it is conventional in marine seismic exploration to locate the detectors in a continuously towed streamer cable, which extends from the rear of a towing vessel, and to detect the acoustic waves, reflected from subsurface formations, while the vessel is proceeding along a prescribed course over the area of interest. However, such a procedure produces towing noise, and this has a disadvantageous influence upon the quality of the signals to be obtained.
To improve both the signal strength and the signal shape it has been proposed to use a plurality of seismic sources belonging to a so-called "compact" array, in particular, a "compact" array of airguns, and it has been found advantageous in marine seismic exploration to generate a plurality of acoustic waves from such an array in order to provide a composite acoustic wave of satisfactory amplitude and frequency content. Airguns having various volume capacities are generally used in such arrays in order to produce a composite acoustic wave having a broad frequency band, since airguns of various volumes generate acoustic waves with different frequency spectrum and corresponding phase spectrum. The frequency content and, in particular, the fundamental frequency of the generated acoustic wave is dependent upon the volume capacity of the airgun.
Therefore, to provide a full spectrum of high energy acoustic pulses it is desirable to generate a number of acoustic waves of various fundamental frequencies. Several arrays have been designed and implemented using an increasing number of guns and an increasing amount of compressed air power. The source strength and the signature shape of these "compact" arrays have been gradually brought to an adequate level. However, problems associated with noise appearance and loss of useful acoustic wave energy in water still exist and are not solved satisfactorily by the above-mentioned "compact" array prior art. For example, meaningful information contained by seismic waves directly reflected from the ocean bottom is often masked due to the reception of horizontally traveling noise caused by reflections of horizontal seismic waves from underwater obstacles and the like. High frequency radiation in more horizontal directions should be suppressed to avoid water trapped noise from dominating the seismic record. However, after this it might well be that still a significant amount of additional energy is needed to overcome the streamer noise and to compensate for nonelastic losses.
Therefore, so-called "extended" airgun arrays are designed to overcome the above-mentioned noise problems. Examples of such "extended" airgun arrays are the so-called superlong airfun array and superwide airgun array. These arrays are known to those skilled in the art and will not be described in detail. In particular, these "extended" arrays provide seismic signals, which have an unsatisfactory signature shape. The signature shape can, for example, be characterized by the primary-to-bubble ratio. The term "primary-to-bubble" ratio (P/B ratio) is known to those skilled in the art and can be defined as the ratio between the amplitudes of the first two significant signal-peaks of the received signal. Conventional extended arrays such as superlong airgun arrays and superwide airgun arrays provide P/B ratios of less than six. However, presently a P/B ratio of more than eight is required.
Accordingly, it is an object of the present invention to provide a method for marine seismic exploration to be used with extended marine seismic arrays, which provides seismic signals of high quality. More particularly, it is an object of the method according to the present invention to provide an improved primary-to-bubble ratio and signature shape of the seismic signals.